Virtually nothing is known about the metabolism of otoconia. Findings in our study of human temporal bones and drug-treated animals suggest that otoconia are continuously being formed and are subject to attrition and disturbances of morphogenesis. Our present results using Ca45 tracer support this concept. This study concerns the rate of turnover of calcium in the otoconia, and investigations of possible disturbances of the uptake. Radioactive calcium, Ca45 will be given in repeated doses to gerbils which are used because their small size permits a high dose of Ca45 in relation to body weight. After the animals have been sacrificed at different time intervals the entire otolithic membranes are removed by means of microdissection. Samples are weighed and counted by liquid scintillation spectrometry. This method is very sensitive and represents the best quantitative method presently available. The uptake of calcium will be measured in relation to time and total dry weight of the otoconia. Since our preliminary results indicate uptake of Ca45 in the so-called "dark cells" in the wall of the utricle, the incorporation of Ca45 in these cells will be investigated. Autoradiography will be carried out in an attempt to localize areas of activity. The effect of vestibulo-toxic drugs on the possible turnover of calcium will be studied by administering streptomycin and gentamicin in appropriate doses. In a final phase we plan to study the calcium uptake in animals with negative calcium balance (hyperparathroidism, rickets, possibly immobilization and senility)in order to find out how the otoconia relate to the body calcium pool. Normal otoconia and abnormal otoconia found in temporal bones from patients with congenital and hereditary hearing loss are being analyzed by powder X-ray diffraction and electron microprobe analyzer. The presence of trace elements in these otoconia will be determined and an attempt will be made to demonstrate differences in composition between central and peripheral regions within the otoconium. The study has basic importance for the clinical otologist. Since man now is exposed to extended periods of weightlessness and to the resulting disturbance of calcium metabolism, it should help to illuminate some of the problems of space medicine.